The present invention relates to digital circuits, and in particular, to a circuit and method for adjusting a digitally controlled oscillator.
FIG. 1 illustrates a communication system 100 that uses digitally controlled oscillator (DCO) 101 to provide a carrier signal for a receiver mixer 103 and a transmitter mixer 104. An input radio frequency (RF) signal RFin is amplified by amplifier 105, demodulated by receiver mixer 103, and processed by inter-frequency (IF)/baseband processor 102. IF/baseband processor 102 also provides a signal that transmitter mixer 104 modulates, and amplifier 106 amplifies to produce RF signal RFout.
DCO 101 receives a control signal of N-bits and may provide a carrier signal having a frequency f1 to receiver mixer 103 and to transmitter mixer 104. DCO 101 uses the digital control signal (N-bits) to adjust the frequency f1.
When the frequency f1 drifts or may otherwise require adjustment, the DCO 101 may be adjusted to provide a carrier signal having a new frequency and/or frequency adjustment. DCO control bits adjust the frequency of the DCO 101 in discrete steps and a minimum step of frequency may represent the smallest change of frequency f1.
The minimum step may be limited by the minimum capacitor values which may be switched internal to DCO 101. The minimum step may cause a disruption, such as glitches, in the processing of the signal within IF/baseband processor 102 or in a subsequent stage. For example, in FM (frequency modulation) demodulation of an audio signal, these glitches may cause clicking sounds from a speaker.
FIG. 2 illustrates the effect of a frequency change 202 on an audio signal 204. FM demodulator 201 receives a FM signal which includes a frequency change 202 over a step 203. This frequency step 203 may create an audio DC step after demodulation, which causes a glitch 205 in the audio signal 204. Glitch 205 may cause audible clicks to the user. If the frequency is recalibrated periodically, it may create undesirable audible tones.